1. Field of the Invention
This invention relates to a wire bonding method for carrying out wire bonding between two conductors.
2. Related Art
To carry out wire bonding between a semiconductor chip and an interconnection, a capillary having an Au wire passing through it is normally used. First, a ball is formed at the tip of the Au wire projecting from the capillary by electric discharge from a torch electrode, and then the capillary is positioned above the semiconductor chip and primary bonding is carried out. After that, the capillary is moved to above the interconnection and secondary bonding is carried out. The semiconductor chip and the interconnection are thereby wire bonded together.
In this case, when the interconnection material is a material whose bondability with Au wire is poor, for example, Cu, Ni or flash Au plating, it is not possible to carry out direct bonding onto the interconnection. For this reason, a substrate of Ag plating or thick Au plating or the like is formed in advance on the area onto which bonding is to be carried out.
However, providing this kind of substrate is not preferable in practice, and a method which makes it possible to carry out direct bonding onto an interconnection even when the interconnection material is one whose bondability with Au wire is poor has been being sought.
Japanese Patent Application Laid-open No. 3-183139 discloses a method wherein wire bonding between a semiconductor chip and an interconnection is carried out by forming a bump on the interconnection in advance by carrying out ball bonding, performing primary bonding onto the semiconductor chip, and then performing secondary bonding onto the bump. With this method, as a result of the bump being formed, it is possible to carry out direct bonding onto an interconnection even when the interconnection material is one whose bondability with Au wire is poor.
However, in this method, when ball bonding is carried out to form the bump, the Au wire is cut from the bump by being pulled upward. Consequently, as shown in FIG. 7A, a tail A forms on the bump 6. Also, when secondary bonding to the bump 6 is carried out, it sometimes happens that as a result of the Au wire 5 being bonded to the tail A on the bump 6, and even larger tail B forms, as shown in FIG. 7B.
When these kinds of tails A and B form, if the tails A, B break, there is a possibility of a short circuit with another interconnection. Also, when the Au wire 5 is bonded to the tail A, as shown in FIG. 7B, the Au wire 5 is cut by being pulled upward. Consequently, dispersion arises in the position at which the Au wire 5 is cut and dispersion arises in the length of the Au wire projecting from the capillary. Since, as described above a ball is formed at the tip of the Au wire by electric discharge between the tip of the Au wire projecting from the capillary and a torch electrode, when there is dispersion in the length of the Au wire projecting from the capillary, dispersion also arises in the diameter of the ball formed at the tip of the Au wire. As a result, the bond strength of wire bonding subsequently carried out changes. Also, when cutting so that no Au wire projects from the capillary no electric discharge occurs between the Au wire and the torch electrode. In a wire bonding apparatus, the electric discharge state of the apparatus is monitored, and when no electric discharge occurs between the Au wire and the torch electrode the apparatus stops.
Thus, the formation of the above-mentioned tails A, B causes various problems.